This invention relates to a switching power converter and, in particular, to a current resonance type DC/DC converter including a resonance circuit and a method of controlling a resonance current thereof.
In the manner which is well known in the art, the DC/DC converter is a switching power converter for converting an input DC voltage (which will later be merely also called an “input voltage”) into an output DC voltage (which will later be merely also called an “output voltage”) which is different from the input DC voltage.
As one of the DC/DC converters, there is a PWM (pulse width modulation) type DC/DC converter is known in the art. The PWM type DC/DC converters have various types which are classified into a step-down type, a step-up type, a polarity reversing type, or the like. The step-down PWM type DC/DC converter comprises an energizing switch, a short-circuit switch, and an output inductor. In lieu of the short-circuit switch, a diode may be used.
However, the PWM type DC/DC converter is disadvantageous in that it has a large switching loss when the energizing switch changes from an on state to an off state or changes from an off state to an on state. As a DC/DC converter which is capable of eliminating such a switching loss, a current resonance type DC/DC converter is known, for example, in U.S. Pat. No. 5,663,635 issued by Vinciarelli et al.
Although the current resonance type DC/DC converter will later be described in conjunction with FIG. 1, the current resonance type DC/DC converter comprises a current resonance type DC/DC converting portion which includes an energizing switch, a resonance inductor, a resonance capacitor, a short-circuit switch, and an output inductor. The energizing switch is turned on/off in response to a first driving control signal. The resonance inductor has an end connected to the energizing switch. The resonance capacitor has an end connected to another end of the resonance inductor and another end which is grounded. The short-circuit switch is connected in parallel with the resonance capacitor. The short-circuit switch is turned on/off in response to a second driving control signal. The output inductor has an end connected to the other end of the resonance inductor and another end connected to an end of an output capacitor.
In the current resonance type DC/DC converter, a current flows through the resonance inductor only for a resonance duration with respect to a switching period. The current does not flow through the resonance inductor for a duration obtained by removing the resonance duration from the switching period. When an input/output voltage ratio becomes smaller, the switching period with respect to the resonance duration becomes longer. As a result, durations where the current does not flow through the resonance inductor increase, as described, for example, in U.S. Pat. No. 4,720,667 issued by Lee et al.
The current resonance type DC/DC converter has a large advantage where a zero-current switching (ZCS) of the energizing switch is enable by using a series resonance of a series resonance circuit consisting of the resonance inductor and the resonance capacitor, and it results in eliminating the switching loss.
In the conventional current resonance type DC/DC converter, a resonance current value is fixed to a value by an input voltage of an input power supply, the resonance inductor, and the resonance capacitor. Therefore, in order to always actualize the zero-current switching (ZCS), it is necessary to always flow, through the resonance inductor, the resonance current having a peak equivalent to a maximum output current value. For example, it will be assumed that the maximum output current value is equal to ten amperes. In this event, it is necessary for the peak of the resonance current have ten amperes or more.
In other words, it is necessary to always flow the resonance current having the peak equivalent to the maximum output current value through the resonance inductor not only on a heavy load where an output current is large but also on no-load or a light load where the output current is small.
In the manner which is described above, it is necessary to always flow the resonance current having the peak equivalent to the maximum output current value through the resonance inductor also on the no-load or the light load where the output current is small. Therefore, on the no-load or the light load, losses become larger caused by the resonance current flowing through the resonance inductor and parasitic resonance components of the energizing switch, the resonance inductor, the resonance capacitor, and so on. As a result, the conventional current resonance type DC/DC converter is disadvantageous in that it has a low degree of efficiency.